Five things you didn't know about your penis

Here are some things you might have wondered about your penis, but were afraid to ask.

1. Your penis does have a mind of its own

You've probably noticed that your penis often does its own thing. You may remember times when it was completely inappropriate to have an erection and yet you couldn't wish it away.

It's true that you have less command over your penis than body parts like your arms and legs. That's because the penis answers to a part of the nervous system that's not always under your conscious control. This is called the autonomic nervous system, which also regulates heart rate and blood pressure.

Sexual arousal usually isn't voluntary. The conscious mind is complicit in it, but a lot of sexual arousal goes on in the sympathetic nervous system. In addition, impulses from the brain during the REM phase of sleep cause erections, whether you're dreaming about sex or about a test you forgot to study for. Heavy lifting or straining to have a bowel movement can also produce an erection.

Just as the penis grows without your consent, sometimes it shrinks. The flaccid penis varies in size considerably.

Exposure to cold water or air makes your penis shrink. That's a function of the sympathetic nervous system.

Psychological stress also involves the sympathetic nervous system, and stress has the same effect as a cold shower. When you're relaxed and feeling well, your flaccid penis looks bigger than when you're stressed.

2. Average penile length and penis length when flaccid and erect

Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.

The NHS says the average penis size is about 14-16cm (5.5-6.3 inches) when erect. The average girth for an erect penis is 12-13cm (4.7-5.1 inches).

A penis is only said to be unusually small if it is less than 3 inches (7.6cm) long when erect.

These measurements were based on a review of 50 studies on penis size conducted since 1942, which between them have measured 11,531 penises.

The NHS also offers tips on measuring your penis, should you be curious. Do with an erection. Take the measurement from the base of the penis, on the side nearest to the tummy and measure up to the tip.

Whatever the clinical significance of these data may be, the changing room significance is considerable. You can't assume that a man with a big limp penis gets much bigger with an erection. Similarly the guy whose penis looks tiny could surprise someone with a big erection.

An analysis of more than thousand measurements taken by sex researcher Alfred Kinsey shows that shorter flaccid penises tend to gain about twice as much length as longer flaccid penises.

A penis that doesn't gain much length with an erection has become known as a "show-er," and a penis that gains a lot is said to be a "grower." These are not medical terms, and there aren't scientifically established thresholds for what's a show-er or a grower.

Kinsey's data suggest that most penises aren't extreme show-ers or growers. About 12% of penises gained one-third or less of their total length with an erection, and about 7% doubled in length when erect.

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